Anomalous properties of water
Water is weird
Water is actually a fairly unusual substance with a number of anomalous properties. For example, most substances contract as they get colder and expand as they get warmer. In regular engineering tasks this is expected and we account for it by including expansion joints in our structures (e.g. bridges, rail tracks). As you’ll see below, water behaves differently. Some of the unusual properties of water turn out to be very important. We will only discuss the anomalies that are important for the purpose of this module.Ice is less dense than liquid water
Most liquids contract during freezing. As a result of that contraction, the material becomes denser. An easy way to visualize that would be to imagine taking the same number of molecules and packing them into a tighter space during freezing. If water behaved in this way, ice would be denser than water and ice would sink. We would live in a world without icebergs and the Titanic would have had a smooth crossing (Too much? Too soon?). Instead, when water freezes at 0°C in regular atmospheric pressure, its volume expands by ~9%. The animation below demonstrates how frozen water acquires a regular solid structure and requires more volume. This makes the ice a little less dense than water and ensures that icebergs float.Water of ~4°C is the densest
The normal expectation for water would be that the coldest water would also be the densest (less heat means less vibration of molecules, which in turn means the lowest volume per number of molecules and thus highest density). However, once again, water is different. We know from the section above that ice is less dense than water, so we know that when we melt ice, the meltwater must be denser than the ice. However, as we heat the water, the density does not initially decrease as we expect. From 0°-4°C the density of water increases; it only starts to decrease as we heat it up beyond 4°C. In a mixture of cold water, the densest water, the water at 4°C, will sink to the bottom of that mixture. The implications for this are tremendous. It means that as our freshwater lakes freeze over in the winter, the deepest parts of the lakes tend to remain at 4°C and have a chance to remain unfrozen. Clearly this is a great benefit to aquatic life that is trying to survive the winter.Solids dissolve in Water
We all know that water has the ability to dissolve solids. As human beings who evolved with a liking for sweets, we take advantage of that property by dissolving sugar and CO2 in water (add extra flavour and you have a soft-drink). That solubility, sugar or otherwise, is once again dependent on the temperature of water. For example, in 100g of room temperature water (20°C), one can dissolve 203.9g of sucrose. Raise the water temperature to 95°C and you can more than double that amount to 448.6g of sucrose. But sugar is not what is dissolved in most of Earth’s water. The oceans are salty because the minerals within rocks have broken down over time and they are responsible for the salinity of the water. The simplest mineral one can imagine for this purpose is salt (NaCl) which occurs as a natural substance and is something we like to add to food rather than water. If we dissolve salt in water, the water becomes salty.But on the molecular scale, there is a big difference between sugary and salty water and that is because sugar and salt dissolve very differently. When you place a spoonful of sugar (Sucrose, C12H22O11) into water, individual molecules of sucrose will detach from the sugar crystals and float intact in the water. When you place a spoonful of salt into water, the NaCl of the salt will be split into its components, Na+ and Cl- ions. These ions will float independently of each other in the water.
The break-down of other minerals works similarly to that of salt and many different ions such as K and Mg are released into the water. One important property of these ions is to reduce the freezing point of water. During the winter, we place a variety of salts on our icy streets to reduce the freezing point. This causes the ice to melt at temperatures of less than 0°C. If you are interested in the solubility of salt in water see this Water (data page).
Having ions float about in the water also changes the conductivity of the water. We will see below that this property has been used to make inferences about icy satellites.
Summary of the properties of water
As we have seen above, water can exist in three phases, water, ice and vapour. Which of these phases we may encounter depends on temperature, pressure and the presence of solutes. In addition, we need to remember that water is weird and ice is less dense than water. As we examine the solar system next we’ll keep these properties in mind. This will help us determine where we might find water and what phase it might be in.There are a number of other unusual and important properties of water, such as that it is unusually incompressible, but for the purpose of our investigation of water in the solar system, the three discussed above are amongst the most important. But for a full listing of all 73 anomalous properties of water, you can marvel at this compilation.
Assorted links for the weirdness of water
If you're into the Weirdness of Water here are a few other optional links to it:ArsTechnica from October 2018: Weird water phase “ice-VII” can grow as fast as 1,000 miles per hour
Gizmodo from January 2018: Scientists Make Coldest Liquid Water Ever, And It's Weirder Than They Imagined
ScienceDaily from December 2017:The origin of water's unusual properties found
ScienceDaily from December 2016: Water: Finding the normal within the weird
Gizmodo March 2022: Rectangular ice crystals result of Ice-VIIt, a new water phase might exist on other planets
